Slow relaxation of the magnetization observed in an antiferromagnetically ordered phase for SCM-based two-dimensional layered compounds

Abstract

Two-dimensional layered compounds with different counteranions, [{Mn(salen)}₄C6](BF₄)₂·2(CH₃OH) (1) and [{Mn(salen)}₄C6](PF₆)₂·2(CH₃OH) (2) (salen²⁻ = N,N′-bis(salicylideneiminato), C6²⁻ = C₆H₁₂(COO)₂²⁻), were synthesized by assembling [Mn(salen)(H₂O)]X (X⁻ = BF₄⁻ and PF₆⁻) and C₆H₁₂(CO₂⁻)₂ (C6²⁻) in a methanol/2-propanol medium. The compounds have similar structures, which are composed of Mn(salen) out-of-plane dimers bridged by μ⁴-type C6²⁻ ions, forming a brick-wall-type network of [–{Mn₂}–OCO–] chains alternately connected via C₆H₁₂ linkers of C6²⁻ moieties. The counteranions for 1 and 2, i.e., BF₄⁻ and PF₆⁻, respectively, are located between layers. Since the size of BF₄⁻ is smaller than that of PF₆⁻, intra-layer inter-chain and inter-plane nearest-neighbor Mn⋯Mn distances are shorter in 1 than in 2. The zigzag chain moiety of [–{Mn₂}–OCO–] leads to a canted S = 2 spin arrangement with ferromagnetic coupling in the Mn^III out-of-plane dimer moiety and antiferromagnetic coupling through –OCO– bridges. Due to strong uniaxial anisotropy of the Mn^III ion, the [–{Mn₂}–OCO–] chains could behave as a single-chain magnet (SCM), which exhibits slow relaxation of magnetization at low temperatures. Nevertheless, these compounds fall into an antiferromagnetic ground state at higher temperatures of T_N = 4.6 and 3.8 K for 1 and 2, respectively, than active temperatures for SCM behavior. The spin flip field at 1.8 K is 2.7 and 1.8 kOe for 1 and 2, respectively, which is attributed to the inter-chain interactions tuned by the size of the counteranions. The relaxation times of magnetization become longer at the boundary between the antiferromagnetic phase and the paramagnetic phase.